It is well known that various aluminum compounds can be used as a coagulant in the treatment of water. The most common of these compounds is alum. Notwithstanding the fact that they are more expensive, basic aluminum chlorides ("BAC") have found acceptance as coagulating agents because of their improved efficiency over alum. BAC has several advantages over alum. The BAC coagulants are effective in lower alkalinity water where alum is relatively ineffective, and BAC has a lesser impact on pH reducing the need for lime or caustic treating of water to bring it within the potable range. The coagulating plant operation is improved since with BAC there is a longer period between backwash. See for example U.S. Pat. Nos. 2,858,269, and 3,270,001 incorporated herein by reference. A more recent development has been the introduction of basic aluminum hydroxychloride sulfates ("BACS") for use as coagulating agents in water treatment.
BACS solutions are prepared by introducing di-valent sulfate ion into the BAC structure. Prior art methods for producing BAC are disclosed in U.S. Pat. Nos. 3,497,459; 3,544,476 and 4,981,673 all of which are incorporated herein by reference.
U.S. Pat. No. 3,544,476, discloses a method for preparing BACS by introducing an anion (Y) into the structure of a basic salt compound having the general formula EQU M.sub.n (OH).sub.m X.sub.3n-m
M is a metal of tri or higher valency, X is an anion capable of forming a monovalent acid, 3n is greater than m and the basicity, defined as m/3nx100, is in the range of about 30 to 83%. Y is an anion capable of forming a di or greater valent acid. M can be aluminum, chromium or iron. Where M is aluminum and X is chlorine the basic compound is a BAC.
The anion, Y, is chemically introduced into the basic salt structure in the form of an acid or its soluble metal salt. Examples of Y are the anions of sulfuric, phosphoric, polyphosphoric, chromic, bichromic, carboxcylic and sulfonic acids. X is preferably Cl, but can be I, NO.sub.3 or CH.sub.3 COO. In this process the production of BAC is not an intermediate step along the route to the formation of a BACS.
The range of the ratio Y/M is about 0.015 to about 0.4. A BACS is formed by conducting the BACS forming reaction in the presence a multivalent acid ion in an acid hydrolysis solution, e.g hydrochloric acid of Al. The BACS may be obtained by the separation of an insoluble sulfate produced by the addition of the hydroxide, oxide or carbonate of calcium or barium to a solution of a normal salt of aluminum containing hydrochloric acid and sulfuric acid. the resulting product stream contains at least 10% or more by weight of the insoluble sulfate salt by-product.
U.S. Pat. No. 3,497,459 discloses and claims a method for preparing a basic salt compound having the formula EQU R.sub.n (OH).sub.m X.sub.3n-m
wherein R can be aluminum and X can be Cl, which comprises the digestion of an oxide ore, e.g. bauxite, with a mixed acid system comprising sulfuric acid and hydrochloric acid. The resulting product solution is treated with an insoluble sulfate forming compound, e.g. calcium carbonate, to neutralize the sulfuric acid used. The reaction solution is filtered to remove the insoluble sulfate precipitate. The mother liquor is alleged to be stable.
The prior art has recognized that BAC may be reacted with sulfuric acid to form a basic aluminum sulfate precipitate for ratios of SO.sub.4 /Al greater than 0.08. The '459 patent suggests that their process inherently passes through that reaction mechanism, but in the process of the '459 patent no such precipitate is formed.
U.S. Pat. No. 4,981,673 discloses a method for preparing a BAC which comprises reacting a solution of aluminum sulfate with a slurry comprising calcium carbonate and calcium chloride. The process stream is filtered to remove precipitate. The filtrate comprises a solution of a BACS of the formula EQU Al.sub.n (OH).sub.m (SO.sub.4).sub.k Cl.sub.3n-m-2k
having a basicity, m/3nx100, of about 40% to about 60% and an Al/Cl equivalent ratio, 3n/3n-m-2k of about 2.85 to 5.
While the prior art recognizes that a BACS can be prepared from a previously prepared BAC by introducing a sulfate ion into the bAC structure, it is uniformly agreed that these compositions are not effective in water treatment processes. The processes which do produce effective BACS result in the formation of byproduct precipitate the disposition of which reduces the cost effectiveness of the process as well as creating environmental problems.